The present invention relates to program timing for electrically operated appliances and in particular relates to program timing for microwave cooking appliances.
Heretofore, control of the interval or duration of electrical energization of a microwave cooking device has been effected by employing a set of switch contacts opened and closed by rotation of a cam drum advanced by a mechanism driven from a sub-fractional horsepower timing motor. Typically in such applications, the cam drum is initially rotated manually by the appliance operator to a position representing the desired cooking interval and the manual rotation of the drum closes a set of contacts for starting the timing motor and a set of contacts for energizing the microwave magnetron.
In such an arrangement, the cam drum is thereafter rotated by the timing motor by the amount representing the selected cooking program interval or "time-out"; and, the cam drum is operable to open the contacts for de-energizing the magnetron and cutting off power to the timing motor. Thus, heretofore, upon termination of the cooking program interval, the appliance was automatically turned off by the programmer timer.
However, where a more sophisticated or complex cooking program has been desired, as for example intermitent or variable magnetron operation during the cooking interval, it has been found necessary to employ a plurality of timer motors for activating separate timing programmers alternatively, electronic timing means have abeen employed which included solid state switching relay devices to handle the power required for energization of the magnetron. Such plural motor programmers or electronic timer programmers have been costly to manufacture. Thus, it has been desired to find a simple low-cost technique for expanding the capability of the programmer timer and yet employ only a single timing motor to drive the cam drum for actuating plurality of circuit function control switches.
In particular it has been desirable to find a way or means of providing partial or intermitent energization of a microwave cooking magnetron upon completion of the cooking cycle for purposes of maintaining the cooked food in a warm state. Prior techniques of providing a post-cooking "KEEP-WARM" mode of operation have required additional timing mechanisms or expensive solid state electronic control circuitry. This it has long been desired to provide such a "KEEP-WARM" function for a microwave cooking oven without incurring the cost of replacing the simple single timing motor actuated programmer timer.